How science happens

I really love quantum physics, but not necessarily the entanglement and computation that I write about here at Ars. Think of my interests like this: if two molecules come into proximity and react to form products, how do they do that? Usually there are many different possible reactions, so why do we end up with the products that we observe?

For many molecules and potential reaction products, the energy released or absorbed during the reaction is enough to tell you what you're going to end up with. But there are many, many cases where the energy differences between different products are small enough that more than one product is produced. Or sometimes a catalyst can intervene to allow reactions that, from an energetic perspective, should be very rare.

In any case, I'm not really interested in the bulk reactions and average products. I'm interested in the details: how do the starting quantum states and coherences between these states influence the final reaction product?

Expanding and generalizing on that: since the quantum states of molecules determine their properties, how much control can we exert over those characteristics through quantum manipulations?

I think there are very few physicists and chemists who would argue these are uninteresting questions. The physicists, however, might say, "This has all been done before." After all, electromagnetically induced transparency, self-induced transparency, frozen light experiments, and Bose Einstein condensation are all examples of manipulating quantum states to set the properties of a material. The chemists would point to molecular beam experiments on cold molecules as evidence that we are already studying how the initial quantum state of a molecule influences subsequent reactions.

These things are already being done, and I probably can't add much. But I don't want to add to those examples; I don't want to work on those sorts of materials. I have no interest in making cold dilute gasses of metallic atoms or molecules, or working with crystalline solids dropped to the temperature of liquid helium. And there is no way that I would willingly enter the seventh circle of the hell that is molecular beams.

No, I want to work with liquids and thin frozen layers—and layers that are not necessarily ordered crystals. (At this point, all the physicists and chemists reading this will be laughing, because liquids are absolutely the worst choice to work with.)

Why liquids?

Liquids are interesting, though. They lack the order that makes the behavior of a crystal relatively clean, yet they still retain near-solid densities, meaning that individual molecules interact with each other constantly. In a very real sense, liquids represent the messy middle ground between low-density cold gases and crystalline solids.

On Earth, liquids are also where all the most interesting stuff occurs. All the chemical reactions that occur in a cell? That's a liquid environment. Almost all pharmaceutical products are produced in wet-chemical reactions. In fact, many industrial processes are condensed phase reactions. If we can get control of and directly play with the states and coherences of molecules in a liquid phase, then we will have an important new tool. This wouldn't just be useful for studying why things are the way they are, but it would also provide us with new knobs to control the properties of materials. I think that is exciting and worthwhile stuff.

We know this is important—experimental results tell us that coherence is a vital aspect of photosynthesis. But there is a rumbling controversy over speculation that enzymes make use of coherence to help catalyze reactions.

On the applications side, we know that coherence between vibrational states—these are the quantum states that describe the spring-like vibrations of atoms within a molecule—is the source of a lot of interesting optical phenomena, and that controlling their coherence would allow for new imaging techniques. All of this paints a picture of something that is of broad interest. Yet no one has been able to put together the important experiments to do any of it. That is what I want to do.

In which I literally have a vision

The thing is, as nice as this idea is, it doesn't matter at all unless I can come up with a reasonable scheme to make the measurements involved. Sometimes, experiments are blindingly obvious, but on other occasions no one knows how to do them.

I often daydream about what atoms and molecules get up to when no one is looking. Lately, I have been getting flashes of images of the coherence involved as molecules approach each other: I dream of sitting on an atom in a molecule and seeing the electron clouds moving around in response to an approaching molecule. At some point, I snap out of it (the hot water has run out, most likely), thinking, "I would love to see that for real."

These sorts of dreams seem to prime my brain. I'll more likely end up dreaming about the same molecule and the same coherence, but this time there will be a laser, or an electrode, or something else involved that interferes with what is going on. I even end up dreaming about what that instrument could record. I'll end up having the same dream with variations to the external interference over and over again. At some point, the dreams converge into one "ideal" experiment.

The kernel for this particular idea came from a dream about how coherence can appear to have vanished yet may, in fact, still be around. Imagine a liquid with a single molecule that is different from the rest. We hit that molecule with a laser pulse designed to excite a specific vibrational mode—for example, a pulse that sets two carbon atoms in motion so that the bond between them stretches and compresses. Now, because the molecule is in a liquid, it is continuously brushing up against other molecules. Every time it does, that vibrational mode is disturbed: its phase might change, or it might give up some energy to another molecule. This process occurs on time scales that are typically faster than a couple of picoseconds (it depends on a lot of factors).

But there's another process going on. The very motion of those two carbon atoms will set other atoms within the same molecule into motion. So exciting one vibrational mode leads to other vibrational modes being excited in a kind of cascade. This process, called delocalization, is a bit like one oscillator (think a ball on a spring) driving another spring linked to it. All kinds of crazy things can eventually happen, but at least initially, the two oscillators are in phase and coherent with each other.

In my dream, I see this molecule's atoms sequentially begin to vibrate until the vibrations get smaller and finally stop. In the absence of collisions, the phase relationship between these different vibrational modes is not simple, or even predictable. It's deterministic. Now, if I were to shine another laser pulse on it, it could take away some vibrational energy from the molecule in a process called Raman scattering. This slows the vibrations but also extracts information about how the original vibrational wave packet spread out through the molecule—information that can be gathered by measuring the frequency, phase, and amplitude of the scattered light.

Imagine that I can do these measurements. What do I learn? I learn some very specific information about that particular molecule and its environment. For example:

The balance between the importance of collisional effects and internal delocalization

The time scale of delocalization

How much collisions randomize the phase of vibrations

Those are interesting things to know. But we're already aware of a fair bit about this, especially for electronically excited atoms and molecules. What does this "ideal" experiment add, and how does it become a research program?

The first step is to realize that the questions this one experiment can answer change based on the context—the molecules and conditions involved. This is actually a very useful thing: the same basic experiment, conducted under different conditions, will tell us very different things, with different implications and relevance. We can take advantage of this to create a research program that easily generates tasty, PhD-sized chunks of science without needing much in the way of new equipment and development time—they are all based on similar measurements from a similar apparatus. Once you have those measurements working, the projects flow.

This is actually the secret to a lot of research labs: a technique is mastered, and then you make hay for as long as you possibly can with it. In the next installment, we'll go through a few of these specific projects to see what sort of hay I want to make.

I really appreciate that you are writing this series. Too often the actual work of science is never discussed outside of the lab. Even when it is discussed publicly, it is generally years after the fact, and all the blind alleys and practical considerations have been forgotten or ignored.

This reminds me of an anecdote I read about John Carmack and how he sees the world. I think it was in Masters of Doom. Paraphrase: Where a normal person showers and doesn't think anything of it, Carmack would look at the light reflecting off and refracting through each water droplet. In his mind he was writing the code to reproduce that, in-engine. The quote was along those lines. Crazy to see how different people see things. Keep it up Chris!

Excellent article. I understood most of it and I am not a physics guy. Your explanation, with analogies, makes it accessible to the well educated lay person. With some chemistry background (up through Biochem some 30 years ago) I can see the potential for all sorts of uses. Well done!

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.